It had been shown by several investigators that cell-mediated immunity, as measured by in vitro response of lymphocytes to mitogens, is depressed in many patients following radiation treatment for cancer of the lung, breast and lymphoid tissues. Proposed are studies to determine whether the deficit indeed results from the radiotherapy or whether histologic types of the neoplasms, stage of progression, or body burden of the tumor are contributing factors. Further, the question of whether inhibitory factors, which prevent lymphocyte response to mitogens, are being released into the patient's serum during radiotherapy is also being studied. For these studies, whole-blood cultures from patients with various histologic types of neoplasms are being tested to determine lymphocyte responses to the non-specific mitogens phytohemagglutinin, pokeweed mitogen, and concanavalin A at different times during and after radiotherapy. For studies on possible inhibitory factors, plasmas from patients exhibiting poor responses are being added to normal control cultures, and control plasmas to patients cultures. Responses of blood cultures from patients during follow-up examination after radiotherapy are being studied and an effort is being made to correlate recovery of lymphocyte response to clinical course of the disease.